Atomic frequency standards are required for various commercial, scientific and military applications. Atomic standards are necessary for communication systems, such as the national telephone networks (AT&T, etc.), ground stations for the planned satellite based cellular telephone networks, and in fact, for satellite ground stations of all types. All simulcast communication systems such as television and radio networks reference atomic standards for stable signal frequency. Navigation systems such as Loran-C and Navstar Global Positioning Satellite (GPS) systems use atomic clocks in the source and sometimes the user equipment. Long baseline radio astronomy requires an atomic clock at each antenna for synchronization of faint signals received from deep space radio objects. National time and frequency standards laboratories in every industrialized country worldwide maintain ensembles of various stable atomic clocks.
The vast majority of clocks in use today are based on quartz crystal oscillators. Examples include wrist watches, wall clocks, etc. There are many reasons why they can be made reliably and inexpensively. Single crystal quartz is piezoelectric so that the mechanical vibration of the quartz is accompanied by a voltage oscillation across the crystal surfaces which is readily measured with simple electrical circuits. Because there is very low conversion of vibrational energy into heat energy, the crystal vibrates at a very sharp frequency. A typical high performance quartz oscillator is a few mm by tenths mm in size and vibrates at 5 MHz. To make a clock from this precision oscillator or frequency standard, the cycles of the vibration are counted electronically so that, e.g., after 5,000,000 cycles have been counted the time reading will be incremented by 1 second, etc.
There are, however, limitations to the stability of the frequency output of even the best of these crystal based clocks. The mounting and packaging of the crystal wafer leads to frequency drifts of a few parts in 10.sup.-12 /day. The oscillation frequency depends on the physical size of the wafer and on the purity of the quartz so that no two wafers can have the same frequency of oscillation.
Atomic frequency standards are set apart from man-made composite resonators, e.g., quartz crystal or cavity resonators, by the inherent indistinguishability of one atom from another, for example, one .sup.133 Cesium atom from another. This quality ensures that atoms as clocks can be more reproducible in their operating frequency than any macroscopic crystal or cavity resonator ever constructed since no two macroscopic man-made resonators will ever be exactly identical. Atoms can absorb radiation of frequency .nu. in going from one energy level to another provided the levels are separated by energy E=hv where h is Planck's constant. Atoms can thus be used as frequency discriminators or filters in their selectivity for absorbing radiation because a slight detuning from the atomic resonance frequency .nu. leads to a slight reduction in atomic absorption. Much of the technology involved in developing atomic clocks thus deals with efficient detection of the atomic resonance.
In typical operation, atomic frequency standards use the stability of an atomic resonance to steer a local oscillator (LO), usually a 5 MHz quartz crystal whose output frequency can be tuned via an input voltage. Because the atomic resonance frequencies for use in clock applications are typically 1 to 40 GHz, the 5 MHz frequency is multiplied up by 1000 or more to match the more stable atomic frequency. The inevitable frequency changes in the crystal are sensed by the atom and converted to an electrical correction signal to steer the crystal frequency so that its multiplied output matches the atomic transition frequency.
The various frequency standards on the market today differ in the choice of atom and the technique used to measure frequency deviations of the LO. The four most common atomic clocks in use today are based on transitions in atomic Hydrogen, Rubidium, Cesium and singly ionized Mercury positive ion. The invention described here pertains to frequency standards based on charged atomic or molecular ions of any sort. However, the preferred embodiment described below uses as an example the .sup.199 Hg.sup.+ atomic ion since in current laboratory practice systems using that ion have shown the most stable clock data.
The primary reason that electromagnetic ion traps are used as the basis for stable frequency standards is that in the environment of a trap, processes that equalize atomic state populations and destroy coherence within the state prepared atomic ensemble are very weak. In the past, coherence times have been measured in an ensemble of trapped Hg.sup.+ ions of over 30 sec on the 40.5-GHz transition. Such weak relaxation has permitted a resonance line width .DELTA..nu. as small as 17 mHz on the 40.5-GHz transition. This line-Q (.nu./.DELTA..nu.=2 .times.10.sup.12) enables good frequency discrimination for local oscillator (LO) fluctuations. [J. D. Prestage, R. L. Tjoelker, G. J. Dick, and L. Maleki, "Ultra-Stable Hg.sup.+ Trapped Ion Frequency Standard", Journal of Modern Optics, vol. 39, pp. 221-232, 1992. ]
One disadvantage of a trapped ion-based frequency standard is the relatively low number of the confined ions. This leads to a low signal-to-noise ratio (SNR) in the detected atomic resonance and consequently limits clock stability. This situation was greatly improved by the use of a Linear Ion Trap (LIT). [J. D. Prestage, R. L. Tjoelker, R. T. Wang, G. J. Dick, and L. Maleki, "Hg.sup.+ Trapped Ion Standard Performance With the Superconducting Cavity Maser Oscillator as L.O.", Proc. 1992 IEEE Frequency Control Symposium, pp. 58-63, 1992.] By use of the LIT, the number of trapped ions was increased by a factor of more than 10 over conventional hyperbolic ion traps disclosed by Leonard S. Cutler et al., "A Trapped Mercury 199 Ion Frequency Standard," Proc. of Thirteenth Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting, NASA Conference Publication 2220, pp. 563-577, Dec. 1-3, 1981.
The stability reached with the improved SNR of the LIT together with the high line-Q has led to a tenfold improvement in clock stability over conventional ion-based clocks. In fact, that LIT-based clock shows stability competitive with the best H-masers for averaging times less than 10,000 sec and exceeds H-maser stability beyond 10,000 sec, making it the most stable of all clocks for long-term stability. However, the LIT-based clock is relatively recent in its development and its configuration continues to evolve. An object of this invention is to provide an improvement in the architecture of the LIT which will lead to improved long-term stability, and a substantial reduction in size, mass and cost of the final frequency standard.
The prior-art LIT-based clock for frequency standard operation is shown in FIG. 1. Ions are created inside a linear trap 10 (shown separately in FIG. 2 to a larger scale) by a pulsed electron beam from a grid electrode source 11. The LIT comprising a trap assembly of four parallel rods 12 spaced as shown schematically in FIG. 3 in an end view of the linear trap 10 which is enclosed in a high vacuum space shown in FIG. 1 as being bounded by magnetic shields 14. (All atomic frequency standards employ magnetic shields to reduce the magnetic field changes over time at the reference atom. Such field changes at the reference atom would shift its energy levels in an uncontrolled way over time and therefore degrade the clock stability.) State selection 194 nm (UV) light from a source comprising a .sup.202 Hg discharge lamp 15 is reflected by a mirror 16 and focused into the central portion of the ion trap 10 through a window 17. Light not absorbed by ions passes through that central portion and is dissipated in a horn 18. Resulting fluorescence from the ions is collected through a window 19 in a direction normal to the sheet of FIG. 2 using an optical detector 20 as shown in FIG. 3.
The pulsed electron beam from the source 11 ionizes a weak vapor of parent neutral atoms introduced into the vacuum space 13 from a heated HgO powder 21 shown in FIG. 1. They are trapped in the space between the ion trap rods 12 via the ponderomotive trapping force (J. D. Prestage et al., "New ion trap for frequency standard applications," J. Appl. Phys. 66, pp. 1013-1017, 1 Aug. 1989) generated by the quadrupole rf bias on the four trap rods. This force confines the ions in the transverse direction. To prevent the ions from escaping along the trap longitudinal axis, a positive bias potential (B+) on end electrodes 22 that are electrically insulated from the trap rods 12 by support elements 23 shown in FIG. 2, and shown in further detail in FIG. 3 of the aforesaid paper of Prestage et al. hereby incorporated by reference. Before the stable atomic frequency of the trapped ions can be compared with the multiplied output of a tuned oscillator 24 shown in FIG. 3, a population difference between the hyperfine levels F=0 and F=1 of the ground state must be created. This preparation is accomplished by optical pumping with the UV light (194 nm) from the lamp 15.
In operation, ions of the neutral vapor of .sup.199 Hg created by the pulsed electron beam from the grid electrode source 11 are held along the ion trap axis. A helium buffer gas (10.sup.-5 mbar) introduced into the vacuum space 13 collisionally cools the ions to near room temperature. The UV light from the .sup.202 Hg discharge lamp 15 optically pumps the ions from the F=1 into the F=0 hyperfine level of the ground state. Thermal motion of the ions along the length of the trap will carry all the ions through the interrogation light field in front of the window 19 indicated in FIG. 2 so that pumping is complete in about 1.5 sec for typical lamp intensities. The lamp 15 is then turned off and tuned microwave (R.F..apprxeq.40.5 GHz) energy from the oscillator 24 is turned on. The microwave energy thus introduced into the interrogation light field 19 via a waveguide 25 and horn 26 shown in FIG. 1 produces resonance with the pumped ions causing them to return to the F=1 level. It is necessary to reduce the UV light level to or near zero during the microwave irradiation period to prevent light shifts and broadening of the clock transition. The microwave energy is then turned off and the pumping lamp 15 is turned on again as shown in FIG. 4 to pump the ions again from the F=1 into the F=0 level, thus producing fluorescence. During the 1.5 second interval following lamp turn-on time, a counter 27 (FIG. 3) is turned on to measure the .sup.199 Hg.sup.+ clock transition back to the F=0 level. A fitted curved line shown in FIG. 5 may be plotted by detection of fluorescence at frequencies above and below a center line at 40,507,348,770 Hz. Measurement of fluorescence accomplished during a 1.5 sec period following the termination of the microwave radiation period is thus a measure of how close the oscillator 24 is tuned to the .sup.199 Hg.sup.+ atomic frequency of 40,507,348,770 Hz.
Ideally, the microwave energy source (oscillator 24) would be tuned to 40,507,348,770 Hz for .sup.199 Hg.sup.+ clock resonance. However, it is not possible to measure the light and maintain oscillator operation with a peak center line precisely that frequency because if it drifts off that frequency so that a different measurement is made during the next cycle of operation it is then not possible to determine the direction of drift in order to introduce a correction in the frequency of the oscillator 24 operating as the microwave energy source. Consequently, the practice is to alternately detune or modulate the microwave frequency that is applied to the ions (as shown in FIG. 5) by +.DELTA..nu. and then -.DELTA..nu. during the next cycle. Any detuning of the oscillator 24 from the center of the atomic resonance frequency 40,507,348,770 will cause a corresponding modulation of the atomic fluorescence during the preparation mode of pumping ions with the microwave signal from the oscillator 24. The frequency of the oscillator is subsequently adjusted to null the difference in light fluorescence obtained at the +.DELTA..nu. and -.DELTA..nu. points. The condition of null fluorescence difference can only occur when the multiplied output of the oscillator is centered on the atomic resonance since then and only then will frequency detuning to +.DELTA..nu. and -.DELTA..nu. give equal fluorescence levels on opposite sides of the symmetric curve shown in FIG. 5.
In practice the size of the modulation .DELTA..nu. is chosen to be the frequency offset from the center line to the point of steepest slope of the atomic resonance curve of FIG. 5. This step corresponds approximately to the frequency at which the fluorescence is one half the peak or central value. Note that the oscillator frequency is steered to follow the atomic resonance while the modulation which is applied to the ions is generated by modulating the output of the synthesizer by .+-..DELTA..nu. and mixing this signal with the multiplied output of the LO. The LO is steered but not directly modulated. In that manner, the precise frequency standard is maintained over an extended period of time to obtain stability of the peak center line at 40,507,348,770.+-.2.times.10.sup.-15 Hz for extended averaging times of about 20,000 sec (51/2 hr). Thus, the prior art LIT-based frequency standard shown in FIG. 1 uses the technique of 194 nm(UV) optical pumping in a first preparation and interrogation (measurement) mode, and irradiating with tuned microwave energy in a second resonance mode in order to probe the hyperfine clock transition in .sup.199 Hg.sup.+ ions immediately after switching back into the first mode.
During the microwave radiation period, it is critical that the atomic resonance frequency not be perturbed by any changes in the trap environment. Such fluctuations would be transferred to the local oscillator, thereby degrading clock stability. Because the preparation (state selecting) UV light will shift the atomic clock resonance, it is switched off during the resonance (microwave energy radiation) mode. Thus, immediately following the microwave energy radiation, the UV lamp is turned on again as shown in the timing diagram of FIG. 4 to determine the extent the microwave radiation has changed the population of the hyperfine levels of the atomic ions during the +.DELTA..nu. and -.DELTA..nu. successive cycles of the dual mode. Any frequency detuning of the oscillator 24 from the reference atomic frequency will change the fluorescent light intensity measured when the UV lamp is turned on.
These fluorescence changes are converted to a voltage and fed back by a computer 24a to a frequency control input of a crystal oscillator 24b to keep it on the frequency of the .sup.199 Hg.sup.+ transitions. The crystal oscillator provides a 5 MHz output which drives a frequency multiplier 24c and a 7.348XXX MHz synthesizer 24d, respectively. The outputs of the multiplier and synthesizer are combined in a mixer 24e to provide a microwave frequency output to be compared to the atomic frequency .nu.. The synthesizer is used to introduce the .+-..DELTA..nu. modulation. Both the crystal oscillator and the synthesizer are controlled by the computer 24a for correction of the microwave frequency standard desired out of the crystal oscillator 24b which is then provided as an output to the user. Thus, the computer corrects the output frequency of the crystal oscillator based on the outcome of the atomic fluorescence measurements and offsets the synthesizer by +.DELTA..nu. and -.DELTA..nu. to points of steepest slope on each side of the atomic resonance curve shown in FIG. 5 for successive measurement cycles. In that manner the computer 24a may determine not only the extent the crystal oscillator has drifted from the precise frequency desired as an output to a user, such as 5 MHz, but also the direction of drift so that an error voltage signal may be produced via a digital to analog (D/A) converter 24f to bring the crystal oscillator back to the desired output frequency to within .+-.2.times.10.sup.-15.
It is apparent from the foregoing discussion on the timing diagram of FIG. 4 that the singular ion trap of FIG. 2 is operated in two modes in the process of controlling the crystal oscillator 24b. In the first mode, with the UV lamp on, the atomic ions are prepared for microwave frequency comparison with the multiplied local oscillator frequency output. In the second mode, with the microwave signal on, the atomic frequency is compared with the microwave signal frequency. During the first (preparation) mode, there are no stringent requirements on environmental isolation or regulation, whereas during the second (resonance) mode, great care must be exercised in regulation of the atomic environment to assure stable atomic frequency operation. It would be desirable to carry out the two modes of (1) preparation of the atomic ions for frequency comparison to the microwave signal frequency and (2) comparison of the microwave frequency to the atomic frequency using separate ion traps implemented as two separate regions of the present invention as described below with advantageous relaxation of many of the constraints of the prior-art LIT.